This invention relates generally to an electrophotographic printing machine, and more particularly concerns a multi-frequency screen for modulating a light image of a color transparency being reproduced by an electrophotographic printing machine.
In the process of electrophotographic printing, a photoconductive member is charged to a substantially uniform level. A light image of the original document irradiates the charged photoconductive member dissipating selectively the charge thereon in accordance with the intensity theeof. In this manner, an electrostatic latent image is recorded on the photoconductive member corresponding to the original document being reproduced. Generally, heat settable particles are employed to develop the latent image. These particles are then trasferred from the latent image to a sheet of support material, in image configuration. Heat is then applied to the particles to permanently affix them to the sheet of support material.
Multi-color electrophotographic printing is substantially the same as the process heretofore discussed. However, a plurality of cycles are employed. Each cycle reproduces a different color contained in the original document. This requires that the light image of the original document be filtered to record an electrostatic latent image corresponding to a single color of the original document. These latent images are developed with appropriately colored particles. The particles are then transferred to the sheet of support material, in superimposed registration with one another. In this manner, a multi-layered powder image is formed on the sheet of support material. This multi-layered powder image is permanently affixed to the sheet of support material by the application of heat to produce a permanent color copy of the original document.
Heretofore, it has been difficult to produce copies having subtle variations of tone or color. Thus, the reproduction of color slides having pictorial quality has not been very feasible. In order to overcome this problem, a half-tone screen is frequently interposed into the optical light path. This screen produces tone gradations by forming half-tone dots or lines of varying size. In the highlight zones, the dots are small increasing in size throughout the intermediate shades until they merge together in the shadow regions. At the highlight end of the tonal scale there will be complete whiteness, while the shadow end will have nearly solid blackness. Numerous patents describe the concept of screening. Exemplary of these patents are U.S. Pat. Nos. 2,598,732; 3,535,036; 3,121,010; 3,193,381; 3,776,633; and 3,809,555.
In addition to the generally available commercial copying machines arranged to reproduce opaque copies, many types of machines are in wide use for reproducing microfilm. For example, U.S. Pat. Nos. 3,424,525; 3,542,468; and 3,547,533 describe typical microfilm copying machines. However, in microfilm copying machines, it has been extremely difficult to form copies of transparencies wherein the copy will have pictorial quality.
With the advent of multi-color electrophotographic printing, it has become highly desirable to be capable of reproducing color transparencies, such as 35mm slides. However, it is required that the copy produced therefrom be of pictorial quality. This necessitates the use of a half-tone screen to achieve this result. One type of system employing half-tone screen for the reproduction of color transparencies is described in co-pending U.S. application, Ser. No. 540,617 now U.S. Pat. No. 4,027,962 filed in 1975. As described therein, a light image of the color transparency is projected through a half-tone screen having 85 dots per inch. However, this may range from about 65 to 300 dots per inch. In this type of a screen, the dot frequency appears to be fixed and only one dot frequency is employed. It has been found that a screen of this type may cause significant light loss due to the relatively high minimum density. In addition, this screen is expensive because of the accurate exposure and development required to produce the required gray scale. In order to improve the foregoing situation, multiple dot line frequencies on a common screen have been employed. Screens of this type show an efficiency gain of more than 100% over a single frequency screen. Moreover, the cost of materials and the control required to make a screen of this type is significantly less than that required to construct a conventional screen.
Heretofore, half-tone cross-lined contact screens have been used to convert an image having a variety of continuous tones into differently sized dots for the preparation of half-tone printing plates. For example, U.S. Pat. No. 3,275,445 issued to Middlemiss in 1966 discloses a screen having a continuous tone medium with portions thereof varying in density. This patent describes the formation of the screen by successive exposure through two cross-lined screens into a continuous tone material. U.S. Pat. No. 2,095,015 issued to VonKujawa in 1937 describes a clear-opaque pattern which is employed through a softly focused lens to manufacture a half-tone screen. The final screen is also produced upon a continuous tone material and has a continuous density range. Similarly, U.S. Pat. No. 3,258,341 issued to Riemerschmid et al. in 1966 and U.S. Pat. No. 3,095,909 issued to Bennett in 1937 teach the use of continuous tone material requiring variable density in their half-tone screens.
It is a primary object of the present invention to improve the screen employed in the optical system of an electrophotographic printing machine reproducing a color transparency.